1. Field of the Invention
The present invention relates to a liquid crystal display, and, more particularly, to a semi-transmission type liquid crystal display which reflects incident light coming from outside to provide a display light source and transmits light from a light source at the back.
2. Description of the Related Art
There is a reflection type liquid crystal display (LCD) known which has a reflector inside and reflects incident external light by the reflector to provide a display light source, thereby eliminating the need for a backlight as a light source and a transmission type liquid crystal display equipped with a backlight as a light source.
As the reflection type liquid crystal display can be designed with lower power consumption, thinner and lighter than the transmission type liquid crystal display, it is mainly used for a portable terminal. This is because as light input from outside is reflected at the reflector in the display, the light can be used as a display light source, thus eliminating the need for a backlight. The transmission type liquid crystal display has such a characteristic as having a better visibility than the reflection type liquid crystal display in case where ambient light is dark.
The basic structure of the existing liquid crystal displays comprises a liquid crystal of an TN (Twisted Nematic) type, a single sheet polarizer type, an STN (Super Twisted Nematic) type, a GH (Guest-Host) type, a PDLC (Polymer Dispersed Liquid Crystal) type, a cholesteric type or the like, a switching element which drives the liquid crystal and a reflector or backlight provided inside or outside a liquid crystal cell. Those ordinary liquid crystal displays employ an active matrix drive system which can achieve high definition and high image quality using thin film transistors (TFTs) or metal/insulating film/metal structure diodes (MIMS) as switching elements, and are equipped with a reflector or backlight.
As a liquid crystal display which has advantages of both the conventional reflection type liquid crystal display and transmission type liquid crystal display, a semi-transmission type liquid crystal display is disclosed (see Japanese Patent No. 2955277) which, as shown in FIG. 1, has gate interconnections 2 and source interconnections 3 so provided as to run around pixel electrodes 1 of an active matrix substrate and intersect each other perpendicularly, has thin film transistors 4 provided on the pixel electrodes 1, has the gate interconnections 2 and source interconnections 3 connected to the gate electrodes and source electrodes of the thin film transistors 4 and has reflection areas 5 of a metal film and transparent areas 6 of ITO formed in the pixel electrodes 1.
As the reflection areas and transparent areas are provided in the pixel electrodes, the backlight can be turned off when the ambient light is bright so that the liquid crystal display can be used as a reflection type liquid crystal display, and thus demonstrates lower power consumption that is the characteristic of the reflection type liquid crystal display. When the ambient light is dark, the backlight is turned on so that the liquid crystal display is used as a transmission type liquid crystal display, and thus demonstrates an improved visibility in case where ambient light is dark, which is the characteristic of the transmission type liquid crystal display. Hereunder, a liquid crystal display which can be used as a reflection type liquid crystal display and as a transmission type liquid crystal display will be called as a semi-transmission type liquid crystal display.
According to the conventional semi-transmission type liquid crystal display, however, incident light travels through the liquid crystal layer back and forth in the reflection area 5 and passes the liquid crystal layer in the transparent area 6, thus producing a difference in light path in the liquid crystal layer. This results in a retardation difference between both areas, which disables the maximization of the intensity of the output light. To solve the problem, the liquid crystal display described in Japanese Patent No. 2955277 has an insulating layer 8 provided under an transparent electrode 7 in the reflection area 5 and a reflector 9 arranged over or under the insulating layer 8, as illustrated in a cross-sectional view of a liquid crystal display shown in FIG. 2, thereby providing a difference between the thickness, dr, of the liquid crystal layer in the reflection area 5 and the thickness, df, of the liquid crystal layer in the transparent area 6.
FIG. 5 is a graph showing the results of computing the intensity, Ip, of the output light in transmission mode and the intensity, Ixcex, of the output light in reflection mode. It is apparent that the intensities of the output light in transmission mode and in reflection mode differ depending on the thickness of the liquid crystal layer. The difference in light path between the reflection area 5 and the transparent area 6 is canceled to approximate the characteristic of the output light by setting the ratio of the thickness dr of the liquid crystal layer in the reflection area to the thickness dr of the liquid crystal layer in the transparent area to about 1:2. Because the thickness of the insulating layer 8 is about a half the thickness of the liquid crystal layer and should be several micrometers, the number of the fabrication processes is increased, thus impairing the planarization of the transparent electrode 7. An alignment film which is formed on the transparent electrode 7 in order to align the liquid crystal molecules is affected by the planarization of the transparent electrode 7. This brings about a problem of making effective alignment difficult in a rubbing process.
Further, as shown in FIG. 3, a step between the reflection area 5 and the transparent area 6 disturbs an electric line of force 10 produced between a lower substrate 11 and an opposite substrate 12, thus deteriorating the characteristics of the liquid crystal display. Furthermore, as shown in FIG. 4, in a liquid crystal layer 13 around the step portion between the reflection area 5 and transparent area 6 on the lower substrate 11, the relationship between the direction of alignment of the liquid crystal molecules and the pretilt angle of the liquid crystal molecules in the vicinity of the surface of the lower substrate 11 generates disturbance in the rotational direction of the liquid crystal molecules (reverse tilt disclination) at the time the liquid crystal display is operated, thus deteriorating the characteristics of the liquid crystal display.
Accordingly, it is an object of the invention to provide a semi-transmission type liquid crystal display which maximizes the luminance in reflection mode as well as transmission mode, so that the alignment of liquid crystal molecules is not disturbed even around the boundary between the reflection area and the transparent area.
A liquid crystal display according to the invention comprises:
a lower substrate on which interconnections and thin film transistors are formed;
an opposite substrate so arranged as to face the lower substrate;
a liquid crystal layer sandwiched between the lower substrate and the opposite substrate;
a reflection electrode formed in a reflection area of the lower substrate;
a transparent electrode formed in a transparent area of the lower substrate;
a common electrode formed on the opposite substrate; and
a drive circuit for applying a voltage between the reflection electrode and the transparent electrode and the common electrode,
whereby a potential difference between a drive voltage applied to that surface of the lower substrate which contacts the liquid crystal layer and a drive voltage applied to that surface of the opposite substrate which contacts the liquid crystal layer is made lower in the transparent area than in the reflection area.
As the drive voltage applied to the liquid crystal layer in the transparent area is lower than the drive voltage applied to the liquid crystal layer in the reflection area, the birefringence of the liquid crystal layer in the reflection area becomes smaller than the birefringence of the liquid crystal layer in the transparent area, making it possible to ensure the optimal birefringence in each of the reflection mode and transmission mode. This can optimize the intensities of the output light in both modes.
The liquid crystal display may be constructed in such a way that the potential difference between the drive voltage applied to that surface of the lower substrate which contacts the liquid crystal layer and the drive voltage applied to that surface of the opposite substrate which contacts the liquid crystal layer is made lower in the transparent area than in the reflection area by capacitive division of an electrostatic capacity of the reflection area.
The capacitive division of the electrostatic capacity of the reflection area produces a difference between the drive voltages of the transparent area and reflection area, so that the transparent area and reflection area can be simultaneously driven by the different voltages using a voltage that is supplied by a single thin film transistor. This makes it possible to prevent an increase in the quantity of the thin film transistors and eliminate the complexity of the drive voltage control, leading to a reduction in the production cost of the liquid crystal display.
The liquid crystal display may be constructed in such a way that a cell gap which is a thickness of a liquid crystal layer in the transparent area is substantially equal to a cell gap which is a thickness of a liquid crystal layer in the reflection area.
As the cell gaps in the transparent area and the reflection area are substantially identical, it is possible to eliminate an alignment disturbance produced by the disturbance of the electric line of force in the liquid crystal layer or an alignment disturbance, such as the reverse tilt disclination produced by the disturbance of the pretilt angle. This can improve the characteristics of the liquid crystal display.
The liquid crystal display may be constructed in such a way that an insulating layer is deposited on the thin film transistors, the reflection electrode and the transparent electrode are formed on the insulating layer at predetermined regions, the transparent electrode is electrically connected to a source electrode of each of the thin film transistors via a contact hole formed in the insulating layer, and the opposite substrate is connected to the transparent electrode via an insulating film.
The connection of the reflection electrode to the transparent electrode via the insulating film allows a capacitor to be formed by the reflection electrode and transparent electrode and a potential difference can be produced between the transparent area and reflection area by capacitively dividing the capacitor formed by the liquid crystal sandwiched between the transparent electrode and the opposite electrode into a capacitor formed by the transparent electrode-insulating film-reflection electrode and a capacitor formed by the reflection electrode-liquid crystal-opposite electrode.
The liquid crystal display may be constructed in such a way that an insulating layer is deposited on the thin film transistors, the reflection electrode and the transparent electrode are formed on the insulating layer at predetermined regions, the transparent electrode is electrically connected to a source electrode of each of the thin film transistors via a contact hole formed in the insulating layer, the opposite substrate is electrically connected to the transparent electrode and an insulating film is deposited on that surface of the opposite substrate which contacts the liquid crystal layer.
As the insulating film is deposited on the reflection electrode, a capacitor formed by the liquid crystal sandwiched between the reflection electrode and the opposite electrode is capacitatively divided into a capacitor formed by the insulating film and a capacitor formed by the liquid crystal, thereby providing a potential difference between the transparent area and the reflection area.
The liquid crystal display may be constructed in such a way that an insulating layer is deposited on the thin film transistors, the reflection electrode and the transparent electrode are formed on the insulating layer at predetermined regions, the transparent electrode is electrically connected to a source electrode of each of the thin film transistors via a contact hole formed in the insulating layer, the opposite substrate is electrically connected to the transparent electrode and an insulating film is deposited on that region of the opposite substrate which faces the reflection electrode.
As the insulating film is deposited on that region of the opposite substrate which faces the reflection electrode, a capacitor formed by the liquid crystal sandwiched between the reflection electrode and the opposite electrode is capacitatively divided into a capacitor formed by the liquid crystal and a capacitor formed by the insulating film, thereby providing a potential difference between the transparent area and the reflection area.
The liquid crystal display may be constructed in such a way that an insulating layer is deposited on the thin film transistors, the reflection electrode and the transparent electrode are formed on the insulating layer at predetermined regions, the transparent electrode is electrically connected to a source electrode of each of the thin film transistors via a contact hole formed in the insulating layer, the opposite substrate is electrically connected to the transparent electrode and an insulating film is deposited on the reflection electrode and that region of the opposite substrate which faces the reflection electrode.
As the insulating film is deposited on the reflection electrode and that region of the opposite substrate which faces the reflection electrode, a capacitor formed by the liquid crystal sandwiched between the reflection electrode and the opposite electrode is capacitatively divided into a capacitor formed by the insulating film and a capacitor formed by the liquid crystal, thereby providing a potential difference between the transparent area and the reflection area.
The liquid crystal display may be constructed in such a way that an insulating layer is deposited on the thin film transistors, the reflection electrode and the transparent electrode are formed on the insulating layer at predetermined regions, the transparent electrode is electrically connected to a source electrode of each of the thin film transistors via a contact hole formed in the insulating layer, a second source electrode is connected to the source electrode via an insulating film, and the reflection electrode is electrically connected to the second source electrode via a contact hole formed in the insulating layer.
As the second source electrode is connected to the source electrode via the insulating film, a capacitor is formed by the reflection electrode and the transparent electrode. By capacitatively dividing a capacitor formed by the liquid crystal sandwiched between the transparent electrode and the opposite electrode into a capacitor formed by the transparent electrode-insulating film-second source electrode and a capacitor formed by the reflection electrode-liquid crystal-opposite electrode, a potential difference can be provided between the transparent area and the reflection area.
The liquid crystal display may be constructed in such a way that an insulating layer is deposited on the thin film transistors, the transparent electrode is formed on the insulating layer, an insulating film is deposited on the transparent electrode, the reflection electrode is formed the insulating film, the transparent electrode is electrically connected to a source electrode of each of the thin film transistors via a contact hole formed in the insulating layer, and openings are formed in the reflection electrode and the insulating film to the transparent electrode.
As the insulating film is formed on the transparent electrode and the reflection electrode is formed on the insulating film, a capacitor is formed by the reflection electrode and the transparent electrode. By capacitatively dividing a capacitor formed by the liquid crystal sandwiched between the transparent electrode and the opposite electrode into a capacitor formed by the transparent electrode-insulating film-reflection electrode and a capacitor formed by the reflection electrode-liquid crystal-opposite electrode, a potential difference can be provided between the transparent area and the reflection area. Because the reflection electrode and insulating film are eliminated at the opening, the opening serves as the transparent area.
The liquid crystal display may be constructed in such a way that undulations are formed on the insulating layer and the openings are formed in top peripheral regions of the undulations and/or bottom peripheral regions thereof.
It is difficult to efficiently reflect light input from the opposite substrate toward a viewer in the top peripheral regions and the bottom peripheral regions of the undulations. Therefore, the openings are formed in the top peripheral regions and the bottom peripheral regions as transparent areas, so that efficient liquid crystal display can be ensured in reflection mode as well as transmission mode.
The insulating film may be formed of one selected from SiN, SiO2, Ti2O3, Ta2O5, SiO, Al2O5, acryl and arton.
Because SiN, SiO2, Ti2O3, Ta2O5, SiO, Al2O5, acryl and arton can be used as the material for the insulating film, it is possible to select the optimal insulating film in accordance with various conditions, such as the usage, the product quality and the material for the liquid crystal. This increases the degree of freedom in the design stage.
The liquid crystal display may be constructed in such a way that a first color filter is formed on the opposite substrate, a second color filter is formed on the thin film transistors and the reflection electrode is formed on the second color filter.
As color filters are formed on the opposite substrate and the device substrate, light passes the color filter on the opposite substrate side twice in reflection mode and light passes the color filters on the device substrate and the opposite substrate once each in transmission mode. This can make it possible to reduce a change in color in both modes. It is also possible to respectively set the hues in transmission mode and reflection mode.